Bipyridine-Directed Syntheses of Uranyl Compounds Containing Semirigid Dicarboxylate Linkers: Diversity and Consistency in Uranyl Speciation

Modular Assembly of Isostructural Mixed-Ligand Uranyl Coordination Polymers Based on a Patterning Strategy

Controlling the orderly assembly of molecular building blocks for the formation of the desired architectural, chemical, and physical properties of the resulting solid-state materials remains a long-term goal and deserves to be examined. In this work, we propose a patterning strategy for modular assembly and structural regulation of mixed-ligand uranyl coordination polymers (CPs) through the combination of couples of organic ligands with complementary molecular geometry and well-matched coordination modes. By using a 5-(p-tolyldiazenyl)isophthalic acid ligand (H₂ptdi) with different rigid linear bicarboxylic acid linkers to construct a well-defined ladder-like pattern, five novel isostructural uranyl coordination polymers, [(UO)₂(ptdi)(bdc)_0.5](dma) (1), [(UO)₂(ptdi)(bpdc)_0.5](dma) (2), [(UO)₂(ptdi)(tpdc)_0.5](dma) (3), [(UO)₂(ptdi)(ndc)_0.5](dma) (4), and [(UO)₂(ptdi) (pdc)_0.5](dma) (5) {H₂bdc, 1,4-dicarboxybenzene; H₂bpdc, 4,4'-biphenyldicarboxylic acid; H₂tpdc, terphenyl-4,4″-dicarboxylic acid; H₂ndc, 2,6-naphthalenedicarboxylic acid; H₂pdc, 1,6-pyrenedicarboxylic acid; [dma]⁺, [(CH₃)₂NH₂]⁺}, were successfully synthesized. Structural analysis reveals that 1-5 have similar ladder-like units but different sizes of one-dimensional nanochannels and interlayer spacing due to the different lengths and widths of the linkers. Because of the changes in interlayer spacing of these isostructural cationic frameworks, differences in the performance of Eu³⁺ ion exchange with [dma]⁺ are observed. Moreover, those compounds with high phase purity have been further characterized by thermogravimetric analysis, infrared spectroscopy, and luminescence spectroscopy, element analysis, PXRD and UV spectroscopy. Among them, compound 3 with strong fluorescence can selectively detect Fe³⁺ over several competing metal cations in aqueous solution. This work not only provides a feasible patterning method for effectively regulating the modular synthesis of functional coordination polymers but also enriches the library of uranyl-based coordination polymers with intriguing structures and functionality.

2,5-Thiophenedicarboxylate: An Interpenetration-Inducing Ligand in Uranyl Chemistry

Seven uranyl ion complexes have been crystallized under solvo-hydrothermal conditions from 2,5-thiophenedicarboxylic acid (tdcH₂) and diverse additional, structure-directing species. [UO₂(tdc)(DMF)] (1) is a two-stranded monoperiodic coordination polymer, while [PPh₃Me][UO₂(tdc)(HCOO)] (2) is a simple chain with terminal formate coligands. Although it is also monoperiodic, [C(NH₂)₃][H₂NMe₂]₂[(UO₂)₃(tdc)₄(HCOO)] (3) displays an alternation of tetra- and hexanuclear rings. Two-stranded subunits are bridged by oxo-coordinated Ni^II cations to form a diperiodic network in [UO₂(tdc)₂Ni(cyclam)] (4), but a homometallic sql diperiodic assembly is built in [Cu(R,S-Me₆cyclam)(H₂O)][UO₂(tdc)₂]·H₂O (5), to which the counterion is hydrogen bonded only. Diperiodic networks with the hcb topology are formed in both [Zn(phen)₃][(UO₂)₂(tdc)₃]·2H₂O·3CH₃CN (6) and [PPh₄]₂[(UO₂)₂(tdc)₃]·2H₂O (7). The slightly undulating layers in 6 are crossed by oblique columns of weakly interacting counterions in polythreading-like fashion. In contrast, the larger curvature in 7 allows for three-fold, parallel 2D interpenetration to occur. These results are compared with previously reported cases of interpenetration and polycatenation in the uranyl-tdc^2- system.

Structural Evolution from Noninterpenetrated to Interpenetrated Thorium-Organic Frameworks Exhibiting High Propyne Storage

Two thorium-organic frameworks of [Th₆O₄(OH)₄(TFBPDC)₆(H₂O)₆]_n (Th-TFBPDC) and [Th₆O₄(OH)₄(TFBPDC)₄(HCOO)₄(H₂O)₆]_n (Th-TFBPDC-i) constructed from the 3,3',5,5'-tetrakis(fluoro)biphenyl-4,4'-dicarboxylate (TFBPDC^2-) ligand were obtained in a reaction. At an early stage of the reaction, the formation of the three-dimensional (3D) framework of Th-TFBPDC was discovered. At a later stage of the reaction, the complete product of Th-TFBPDC-i was obtained. The structural evolution from a noninterpenetrated network of Th-TFBPDC to a 2-fold interpenetrated network of Th-TFBPDC-i is a dissolution-recrystallization process and rationalized as the four equatorial TFBPDC^2- ligands in an octahedral [Th₆O₄(OH)₄(TFBPDC)₁₂] unit were displaced by four formate ligands to form a [Th₆O₄(OH)₄(TFBPDC)₈(HCOO)₄] unit via a ligand substitution reaction. The large pore volume as well as the strong interactions between the host framework and guest propyne (C₃H₄) molecules demonstrated by computational results endow the highly water-stable Th-TFBPDC with the best-performing C₃H₄ storage under ambient conditions. This work presents a rare example of structural evolution from a 3D noninterpenetrated network to a 2-fold 3D interpenetrated network and a highly promising metal-organic framework (MOF) for C₃H₄ storage with a C₃H₄ uptake of 8.16 mmol g^-1 at 298 K.

Kinked-Helix Actinide Polyrotaxanes from Weakly Bound Pseudorotaxane Linkers with Variable Conformations

The incorporation of a mechanically interlocked molecule such as pseudorotaxane into metal-organic coordination polymers has afforded plenty of new hybrid materials with special structures and unique properties. In this work, we employ a weakly bound cucurbit[6]uril (CB[6])-bipyridinium pseudorotaxane as a supramolecular precursor to assemble with uranyl, aiming to construct uranyl-rotaxane coordination polymers (URCPs) with intriguing structures. By adjusting the synthetic conditions, a new kinked-helix uranyl rotaxane compound (URCP3), together with three other compounds URCP1, URCP2, and URCP4 varying from 1D chains to 2D interwoven networks, was obtained. Detailed structural analyses indicate that the pseudorotaxane ligand (C8BPCA@CB[6]) shows great configuration diversity in the construction of URCPs, which is most probably due to the weak binding strength between the host and guest molecules. Specifically, based on the monodentate coordination of the end carboxyl groups of C8BPCA forced by the surrounding unilaterally-chelated oxalate, the entire flexible pseudorotaxane linker will be more likely to undergo conformational change, thereby binding to the uranyl center from both sides of the uranyl equatorial plane and promoting the formation of a kinked helix structure of URCP3 that is shaped like a Chinese knot along [001]. This work enriches the library of actinide-rotaxane compounds and provides a new approach to construct metal-organic compounds with complicated structures using weakly bonded pseudorotaxanes as well.

Noncomplexed Cucurbituril-Mediated Structural Evolution of Layered Uranyl Terephthalate Compounds

Template synthesis is one of the most feasible ways to explore new uranyl compounds with intriguing structures and properties. Here we demonstrate the preparation of six novel "sandwichlike" uranyl coordination polymers (UCPs) based on two-dimensional uranyl-terephthalate acid (H₂TP) networks using CBn (n = 5, 6, 8) as template ligands in the presence of different cations (Na⁺, K⁺, Cs⁺, or H₂N(CH₃)₂⁺). Compound 1 ([UO₂(TP)₂][Na₂(CB5)(H₂O)](H₂O)₅) is composed of layered uranyl-TP networks with the complex of CB5 and sodium cations as template ligands. In compound 2 ([(UO₂)₂(TP)₃]₂(CB6)(H₂O)₁₀), CB6 located between uranyl-TP networks contacts them by π-π interactions and hydrogen bonds. Compound 3 ([(UO₂)₂(TP)₃]₂[Na₂(H₂O)₁₀(CB6)]) is the same as compound 2 except for sodium cations bonding with CB6. Similarly in compound 4 ([(UO₂)₂(TP)₃][Cs(H₂O)₃(CB6)]), CB6 is a capsulelike structure capped with two cesium cations and interacts with uranyl-TP networks through π-π and C-H···π interactions. Compound 5 ([(UO₂)₂(TP)₃(HCOO)₂][K(H₂O)₂(CB5)]₂[H₂N(CH₃)₂]₂(CB6)(H₂O)₆) consists of both templates of CB6 and CB5 in which each CB5 is capped with one potassium cation while the H₂N(CH₃)₂⁺ cation is held at CB6 portals. In compound 6 ([(UO₂)₂(TP)₃]₂[UO₂(TP)₂(H₂O)₂][Cs(CB8)₃(H₂O)₄](H₂O)₁₆), CB8 ligands are connected by cesium cations to form a triangle motif and are further located between the uranyl-TP networks as template agents. All of the 2D layered structures with free CBn or cation-anchored CBn intercalate into the laminates of uranyl-terephthalate and shows a cucurbituril-mediated structural evolution. The regulating role of CBn as structure-directing template agents for the construction of layered UCPs through outer-surface interactions with layers of uranyl terephthalate is demonstrated, especially for the case of CB6 with contractive interlayer spacing.

Occurrence of polyoxouranium motifs in uranyl organic networks constructed by using silicon-centered carboxylate linkers: structures, spectroscopy and computation

Four polyoxouranium-based uranyl carboxylates have been synthesized based on silicon-centered carboxylate linkers. Oligomerization of the uranyl units from tetrameric unit, to octameric motif and ultimately infinite polyoxouranium chain was observed.

A mixed-ligand strategy regulates thorium-based MOFs

A thorium-based MOF formed via the synergistic construction of porphyrin and bipyridyl based on the mixed-ligand strategy has the effect of enhancing photocatalysis.

Zero-, mono- and diperiodic uranyl ion complexes with the diphenate dianion: influences of transition metal ion coordination and differential UVI chelation

Diphenate complexes with uranyl cations are generally of low periodicity (0 or 1), but for one 2-periodic uranyl–Cu^II species.